Hi,
I am also a contractor and I hate to tell you, but I don't think we are entitled to much at all.
especially with you being self-employed.
the most you will probably get is Maternity Allowance - arranged through Job Center.
go here -
https://www.gov.uk/maternity-allowance/overview
there are step by step questionnaire that ask about your situation and can give you an indication of how much you could get.
good luck - I am due at the end of Jan, so will be facing this soon myself!